Magnetic particle imaging (MPI) is an emerging biomedical imaging technology enabled by the non-linear response of magnetic nanoparticle tracers to a scanned magnetic gradient field.
1 This tracer-based technology provides a safer alternative to other radioactive imaging modalities like single photon emission computed tomography (SPECT) and positron emission tomography (PET). MPI has been demonstrated in applications such as real-time cardiovascular imaging
2, cell labeling and tracking
3,4, and has potential in magnetic fluid hyperthermia.
5 The original MPI theory assumes that tracers respond instantaneously to the magnetic fields encountered in MPI. In practice, this is not the case, and finite magnetic relaxation or response time has been implicated in secondary blurring of the image, even limiting the attainable resolution in MPI.
6,7 To understand the effect of response time, or relaxation, on the MPI signal and resolution, we applied rotational Brownian dynamics (RBD) simulations
8 and we observed a drop in resolution and signal. To investigate the effect of field-dependent relaxation we used ferrohydrodynamic modeling to obtain predictions of the point spread function (PSF) in an x-space relaxometer
7 and the nanoparticle harmonic spectra in a magnetic particle spectrometer (MPS)
9,10. These instruments are employed to assess the performance of tracers for use in MPI. Through this study we observed considerable agreement between the model predictions and experiments without the need of empirical parameter fits. Finally, we explored the potential of MPI field gradients in magnetic fluid hyperthermia to achieve selective heating of nanoparticles. Here, we used the ferrohydrodynamic equations to calculate theoretical predictions of specific absorption rate (SAR) and obtain millimeter scale spatial distribution of SAR by tuning the MPI field gradients. Through our modeling efforts we were able to understand the effect of relaxation in MPI and show the potential of MPI field gradients to achieve spatially selective magnetic fluid hyperthermia.
1 B. Gleich and J. Weizenecker, Nature 435,1214 (2005).
2 J. Weizenecker, B. Gleich, J. Rahmer, H. Dahnke, and J. Borgert, Physics in Medicine and Biology 54,L1 (2009).
3 B. Zheng, M. P. von See, E. Yu, B. Gunel, K. Lu, T. Vazin, D. V. Schaffer, P. W. Goodwill, and S. M. Conolly, Theranostics 6,291 (2016).
4 B. Zheng, T. Vazin, P. W. Goodwill, A. Conway, A. Verma, E. Ulku Saritas, D. Schaffer, and S. M. Conolly, Sci Rep 5,14055 (2015).
5 K. Murase, H. Takata, Y. Takeuchi, and S. Saito, Physica Medica-European Journal of Medical Physics 29,624 (2013).
6 L. R. Croft, P. W. Goodwill, and S. M. Conolly, IEEE Transactions on Medical Imaging 31,2335 (2012).
7 P. W. Goodwill, A. Tamrazian, L. R. Croft, C. D. Lu, E. M. Johnson, R. Pidaparthi, R. M. Ferguson, A. P. Khandhar, K. M. Krishnan, and S. M. Conolly, Applied Physics Letters 98,
262502 (2011).
8 R. Dhavalikar and C. Rinaldi, Journal of Applied Physics 115,074308 (2014).
9 R. Dhavalikar, L. Maldonado-Camargo, N. Garraud, and C. Rinaldi, Journal of Applied Physics 118,173906 (2015).
10 S. Biederer, T. Knopp, T. F. Sattel, K. Ludtke-Buzug, B. Gleich, J. Weizenecker, J. Borgert, and T. M. Buzug, Journal of Physics D-Applied Physics 42, 205007 (2009).
